1. Field of the Invention
The present invention relates to display devices, methods for driving the same, and portable terminal apparatuses using the same. More particularly, the present invention relates to a display device employing a selector circuit addressing method for driving the signal lines of a display panel, a method for driving the same, and a portable terminal apparatus provided with the same as a display unit.
2. Description of the Related Art
Display devices having pixels disposed in a matrix include a liquid crystal display using a liquid crystal cell as the pixel. The liquid crystal display employs the passive matrix system or the active matrix system as the driving method. Recently, for its improved response characteristics and visibility characteristics, most liquid crystal displays have employed the active matrix system. When a liquid crystal panel is driven, the active matrix display selects a scan line to be written with a signal and then feeds the signal from, for example, an external driver IC to a signal line. Thus, the signal is written to the pixel in a matrix determined to be driven.
The external driver IC, which is located outside the panel, drives the signal lines of the liquid crystal panel. When there is a one-to-one correspondence between the output of the external driver IC and each of the signal lines, a driver IC with a number of outputs equal to the number of the signal lines must be prepared. In addition, a corresponding number of wires are required for establishing the connection between the driver IC and the liquid crystal panel. In view of this, there is employed a selector circuit addressing method which assigns a plurality of signal lines to an output of the driver IC as a unit (pair), and selects each of the plurality of signal lines in a time-sharing manner so that an output signal from the driver IC is fed into the selected signal line in a time-sharing manner.
In this selector addressing method, one to x correspondences between the outputs of the driver IC and the signal lines of the liquid crystal panel are specifically set, where x is an integer of at least 2, and all of the x signal lines assigned to one output of the driver IC are selected to be driven every x time divisions. Employing this selector addressing method can reduce the number of wiring connections to 1/x.
A liquid crystal display having a built-in drive circuit is constructed by monolithically forming a pixel unit and a drive circuit for driving the pixel unit on a common substrate (liquid crystal panel). When the selector addressing method is employed by this liquid crystal display having a built-in drive circuit, a selection circuit that serves to distribute one output signal from the output of the driver IC to x signal lines in a time-sharing manner is mounted on the liquid crystal panel. The selector switches in response to selector pulses supplied from an external circuit.
Furthermore, a level converter is mounted on the liquid crystal panel. For example, the level converter level shifts a TTL level signal having a low-voltage swing and supplied from the external IC into a signal having a high-voltage swing required for driving the liquid crystal. The above selector pulse, which uses TTL levels having a low-voltage swing (for example, 0 to 3.3 V), is fed into the level converter where the selector pulse is level shifted to a signal having a high-voltage swing (for example, 0 to 7.7 V) required for driving the liquid crystal panel, which is then is supplied to the selection circuit for time-sharing control.
The liquid crystal display changes its molecular arrangement pattern depending on the presence of an electric field, thus performing transmission/blocking control of light for image display. Since the principle of operation of the liquid crystal display allows it to be driven with little power, these displays are widely used as output display units for, particularly, portable telephones and PDAs (Personal Digital Assistants) using a battery as the main power supply. The liquid crystal displays of this type are being developed to operate with low power consumption by decreasing the driving voltage and the driving frequency. This allows the displays to be operated for a long time.
However, wasteful D.C. consumption occurs in the liquid crystal display with the above selector addressing method because the level converter that level-shits the selector pulse for the external-circuit power voltage to the one for the internal-circuit power voltage is constantly maintained in the ON state. This prevents the power consumption of the entire drive circuit from being decreased. When the liquid crystal display is applied, in particular, to portable terminal apparatuses such as portable telephones and PDAs, lower power consumption of the display device, such as a liquid crystal display, is a very important issue for realizing further reduced power consumption in the portable terminal apparatus.